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Perfluorooctane sulfonate, PFOS, was previously widely used in photolithography, but it and other long-chain acids had to be phased out due to concerns about their bioaccumulation potential and toxicity. A number of replacement chemicals are now used, including short-chain and emerging per- and polyfluoroalkyl substances (PFAS) such as perfluorobutane sulfonate (PFBS), fluorinated ethers, and non-PFAS alternatives. A wide variety of replacement chemistries have been used or proposed, ranging from simple small-molecule acids to complex fluorinated polymers. A major challenge in finding suitable alternatives is to balance performance with the need to avoid regrettable substitutions, which can lead to continuing harm to workers, communities, and ecosystems, and leave producers in positions of regulatory uncertainty. Assessment of PFAS continues to be limited by lack of data on hazard properties. Despite decades of growing interest, data on bioaccumulation potential and toxicity exist for only a handful of them. Based on the lack of available data, there is a clear need for scientifically sound assessments of both fluorinated and non-PFAS chemicals if they are to be adopted as alternatives. Yet a chemical-by-chemical approach to such evaluation is both time- and resource-intensive. Our group is developing a molecular dynamics-based framework to evaluate the relative hazards of PFAS and non-PFAS alternatives in comparison with substances like PFOS that are known to be problematic in their bioaccumulation and toxicity profiles. Molecular dynamics is used to predict the strength PFAS association with a variety of proteins selected for their contribution to the bioaccumulation potential and specific toxic mechanisms of PFAS, including fatty acid transport proteins, serum albumin, and thyroid hormone receptors. The individual predicted are then combined within a weighted score framework to rank substances by an overall hazard score (Figure 1). This is similar to existing approaches to evaluate neutral organic substances based on their persistence, bioaccumulation, and toxicity (PBT) profiles, but this is the first such attempt to focus on PFAS-specific properties. Upcoming work will further validate this hazard-ranking scheme using a zebrafish embryo toxicity assay for substances that are available for analysis, compared to benchmark chemicals like PFOS.
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